1. Field of the Invention
The present invention relates generally to an original reading apparatus for performing reading of an original, such as a facsimile apparatus, a copy machine, a scanner and so forth. More particularly, the invention relates to an original reading apparatus using an optical system elongating an optical path length by turning a light beam by means of a mirror.
2. Description of the Related Art
An original reading apparatus has been used not only in office but also in home in applications of electronizing of various materials, taking in image data from an exposed film and so forth. Particularly, as the original reading apparatus used in home or small scale office, compact one is preferred. Such compact original reading apparatus frequently employs a scanner module constructed with a linear image sensor in which a plurality of reading elements are aligned in a primary scanning direction, and a compressing optical system forming an image of a sheet form original or original, such as book or the like on the primary image sensor as compressed image, and a light source, as a unit for reading the image. The scanner module moves within a given casing in an auxiliary scanning direction perpendicular to the primary scanning direction. At this time, a two dimensional image information of the original mounted on a flat plate form platen glass arranged in the upper portion of the casing, is read.
On the other hand, even in such compact original reading apparatus, there is an increasing demand for reading the original with a plurality of magnifications. For instance, if it is possible to set a film on the platen glass and to read an enlarged image, it becomes unnecessary to separately buy a dedicated film scanner. Attempting to obtain images of a plurality of magnifications using the same linear image sensor, it becomes necessary to vary a distance between the original or object to a lens and a distance between the lens and the linear image sensor, and in conjunction therewith, to vary a focal distance of the lens so that the image can be formed on the linear image sensor in the condition where the distances are varied.
FIG. 7 illustrates a major portion of an optical system of the conventionally proposed copy machine disclosed in Japanese Unexamined Patent Publication No. 2001-109079. In the lower side of the platen glass 11, first and second optical scanning portions 12 and 13 arranged for reciprocal motion in lateral direction in the drawing (auxiliary scanning direction), a lens 14 arranged therebetween, a photosensitive drum 15 exposing the image and a sixth mirror 16 guiding a light output from the second optical scanning portion to the photosensitive drum 15.
The first optical scanning portion 12 is constructed with a light source 18 illuminating a linear reading position of the platen glass 11 (direction perpendicular to drawing sheet surface) and first to third mirrors 21 to 23 respectively reflecting a reflected light of the original (not shown) by illumination of the reading position by the light source 18. A light reflected by the third mirror 23 incides to the second optical scanning portion 13 via the lens 14. The second optical scanning portion 13 sequentially reflects the incident light by fourth and fifth mirrors 24 and 25 to incide the output light of the second optical scanning portion 13 to the sixth mirror 16. This arrangement of the optical system shown in FIG. 7 is to form an image of equal magnification (100%) on the photosensitive drum 15.
FIG. 8 is an illustration for explaining a relationship between a magnification and object, lens and the image forming position. Assuming that a distance between an object 31 having a length A and a lens 32 is a, and a distance between the lens 32 and an image 32 having a length B and formed at a focal position of the linear image sensor or the like is b, a magnification B/A of the image 33 can be expressed by a ratio of two distances b/a. Therefore, in the technology shown in FIG. 7, the ratio b/a of the distances is varied by individually moving the first optical scanning portion 12, the second optical scanning portion 13 and the lens 14 for setting various magnification.
FIG. 9 shows the case where a magnification is 50%, and in contrast, FIG. 10 shows the case where a magnification is 200%. As can be seen, position of the lens 14 is relatively shifted laterally (left and right direction in the drawing). By this, the ratio b/a of the distances is varied. Of course, in practical reading of the image, the first optical scanning portion 12 is moved (performs auxiliary scan) from a left side end in the drawing to a right side end relative to the platen glass 11. Associating with this, the second optical scanning portion 13 and the lens 14 are also moved with maintaining positional relationship.
Even in the technology disclosed in Japanese Unexamined Patent Publication No. 06-27539, an optical system in which light beam is sequentially turned by six mirrors, similarly, is prepared, and magnification is varied by shifting the positions of lens and mirror are shifted toward the object side or the image forming side.
On the other hand, in the technology disclosed in Japanese Unexamined Patent Publication No. 06-27539, a plurality of magnifications are stored in storage means, and these magnifications can be set easily. In addition, in the technology disclosed in Japanese Unexamined Patent Publication No. 11-305356, four mirrors are used and magnification is varied by shifting the mounting position of the lens or exchanging the lens, and in conjunction therewith shifting the focal position where the photosensitive material is arranged.
As set forth above, conventionally, in the original reading apparatus, it has been typically known to certainly obtain an optical length by sequentially turning the optical paths at respective mirrors using a plurality of mirrors in order to form the image of the original on the image sensor or the photosensitive body using relatively narrow space. Thus, upon varying magnification in stepwise manner or sequentially in the original reading apparatus set forth above, variation of magnification is achieved by varying the relative position of the lens or mirror or focal position without varying number of turns of the optical path with respect to these mirrors.
Therefore, these shifting mechanism of the original reading apparatus can be complicated. Furthermore, when the magnification of the optical system is varied significantly, shifting magnitudes of respective parts become large. On the other hand, even when a plurality of mirrors used, necessity is caused to certainly obtain sufficient length in auxiliary scanning direction in order to acquire optical path length even when a plurality of mirrors are used. As a result, the overall original reading apparatus becomes bulky to make it impossible form the compact original reading apparatus to employ the mechanism significantly varying the magnification.